A conventional tracer control unit has carried out a tracer control subject so that the axes of a cutter of a copy machine tool are mechanically coupled to a tracer head. More specifically, the tracer control unit detects the amount of deflection of all axes of a stylus by the tracer head and the command values for the axes are calculated using these amounts of deflections in a trace computing circuit. Then with these command values, the cutter is relatively moved to a workpiece by driving the motors for the axes while the tracer head is moved at the same speed along a surface of a model. The tracer control for machining a workpiece in the same shape as the profile of the model is carried out by repeating the above operations.
In the case that a tool such as, for example, a cutter and a workpiece are to be moved in accordance with the tracer command values generated by the conventional tracer control unit, the tracer head and the tool are simultaneously driven with the feed axis drive commands thus generated and the tracer control unit has not been provided with a function for moving the tracer preceding movement of the cutter axis.
However, when the stylus provided on the tip of the tracer head collides with a corner or a similar part of the model during the tracing of the model, the tracer head bites the model for a moment, because the tracer head travels excessively because of effects such as the delay of the servo system that forms the driver of the tracer head and the inertia of the machine.
Consequently, the cutter also bites the workpiece as much as that of the tracer head and the machining accuracy at the corner of the model deteriorates. Accordingly, the conventional tracer control unit has a problem in that a part of the model where the profile is abruptly changed cannot be accurately traced.